The present invention relates generally to multilayer antireflection coatings for substrates, and more particularly to multilayer antireflection coatings deposited on temperature-sensitive substrates by DC reactive sputtering.
The simplest antireflection coating is a single layer of a transparent material having a refractive index less than that of a substrate on which it is disposed. The optical thickness of such a layer may be about one-quarter wavelength at a wavelength of about 520 nanometers (nm), i.e., at about the middle of the visible spectrum. The visible spectrum extends from a wavelength of about 420 nm to a wavelength of about 680 nm. A single layer coating produces a minimum reflection value at the wavelength at which the layer's optical thickness is one-quarter of the wavelength. At all other wavelengths the reflection is higher than the minimum but less than the reflection of an uncoated substrate. An uncoated glass surface having a refractive index of about 1.52 reflects about 4.3 percent of the normally-incident light.
Multilayer antireflection coatings are made by depositing two or more layers of transparent dielectric materials on a substrate. At least one layer has a refractive index higher than the refractive index of the substrate. The layer systems usually include at least three layers and are designed to reduce reflection at all wavelengths in the visible spectrum. Multilayer antireflection coatings may yield reflection values of less than 0.25 percent over the visible spectrum.
Most multilayer antireflection coatings are derived from a basic three layer system. The first or outermost layer of this system has a refractive index lower than that of the substrate and an optical thickness of about one-quarter wavelength at a wavelength of about 520 nm. The second or middle layer has a refractive index higher than that of the substrate and an optical thickness of about one-half wavelength at a wavelength of about 520 nm. The third layer, i.e. the layer deposited on the substrate, has a refractive index greater than that of the substrate but less than that of the second layer. The optical thickness of the third layer is also about one-quarter wavelength at a wavelength of about 520 nm. This basic design was first described in the paper by Lockhart and King, "Three Layered Reflection Reducing Coatings", J. Opt. Soc. Am., Vol. 37, pp. 689-694 (1947).
A disadvantage of the basic three layer system is that the refractive indices of the layers must have specific values in order to produce optimum performance. The selection and control of the refractive index of the third layer is particularly important. Deviation from specific refractive index values can not be compensated for by varying the thickness of the layers.
Various modifications of the Lockhart and King system have been made to overcome these disadvantages. For example, the layer system has been modified by forming at least one layer from mixtures of two materials having refractive indices higher and lower than the desired value for the layer. The refractive index of one or more layers has also been simulated by using groups of thinner layers having about the same total optical thickness as the desired layer, but including layers having refractive index values higher and lower than the desired value.
Other modifications have included varying the refractive index of one or more of the layers as a function of thickness, i.e., having the refractive index of a layer inhomogeneous in the thickness direction. This approach is described in U.S. Pat. No. 3,960,441. Another modification is the use of an additional layer between the basic three layer system and the substrate. This additional layer may have an optical thickness of about one-half wavelength, i.e., about half the thickness of the basic system, and a refractive index less than that of the substrate. This modification is disclosed in U.S. Pat. No. 3,781,090.
The layer systems discussed above are generally deposited by thermal evaporation. In thermal evaporation, the time required to deposit the layers may be only a relatively small fraction of the total production time. The production time may be determined by such factors as pump down time for the coating chamber, the time required to heat substrates to process temperatures, and the time required to cool substrates after coating. The number of layers in the coating, the thickness of the layers, and the layer materials may not have a significant influence on production time and thus cost.
DC reactive sputtering is the process most often used for large area commercial coating applications. Metal oxide layers, for example, are deposited by sputtering the appropriate metal in an atmosphere including oxygen. In the reactive sputtering process, the articles to be coated are passed through a series of in-line vacuum chambers, each including sputtering sources, i.e., sputtering cathodes. The chambers are isolated from one another by vacuum locks. Such a system may be referred to as an in-line system or simply a glass coater.
The time taken to deposit the layers is determined mainly by the number of layers and the sputtering rate of the materials. The use of a glass coater to deposit multilayer antireflection coatings can significantly reduce their cost, extending their range of application. Such coatings may be used on picture frame glass, for a display case, and as thermal control coatings for architectural and automobile glazings.
Many of the materials used in thermal evaporation processes, particularly fluorides and sulfides, are not easily sputtered. Conversely, a few materials, such as zinc oxide (ZnO), commonly used in the architectural glass sputtering systems are rarely, if ever used, in thermal evaporation processes. The sputtering rate of different materials may vary by a factor of greater than twenty. The choice of materials, therefore, can have a significant influence on the deposition time and fabrication cost. In an in-line sputtering system with multiple chambers, each chamber may be set up to deposit one specific material. As such, the number of layers that can be deposited is determined by the number of chambers. A coating designed for sputter deposition should therefore be as simple as possible. It should also be made, if possible, from materials which have a high sputtering rate.
A simple improvement on the Lockhart and King system, which may be suitable for in-line sputtering, is described in U.S. Pat. No. 3,432,225, the entire disclosure of which is hereby incorporated by reference. This system, called the Rock system, includes four layers. The first or outermost layer has a refractive index lower than that of the substrate and an optical thickness of about one-quarter wavelength at a wavelength of about 520 nm. The second or middle layer has a refractive index higher than that of the substrate and an optical thickness of about one-half to six-tenths of a wavelength at a wavelength of about 520 nm. The third layer has an optical thickness of about one-tenth of a wavelength at a wavelength of 520 nm and a refractive index less than that of the second layer. The fourth layer has an optical thickness of about one-tenth of a wavelength and a refractive index greater than the second layer and the substrate. The third layer may be the same material as the first layer, and the fourth layer may be the same material as the second layer.
The Rock system may be used with different combinations of materials. Differences in refractive indices may be compensated for by different layer thicknesses. Specifically, for a selected set of materials, the layer thicknesses of the Rock system may be adjusted to provide optimum performance. Specific refractive index values for the layers are not required. If a higher refractive index material were used for the outer layer, then the refractive index of the second layer would also need to be higher to produce the lowest reflectivity. However, in order to obtain the lowest reflection values, the refractive index of the first and third layers should be less than about 1.5, and the refractive index of the second and fourth layers should be greater than about 2.2. A Rock system suitable for sputtering may use silicon dioxide (SiO.sub.2) with a refractive index of about 1.46 at 520 nm for the first and third layers, and titanium dioxide (TiO.sub.2) with a refractive index of about 2.35 at 520 nm for the second and fourth layers.
Magnesium fluoride (MgFl) can be used to form the outer and third layers. Magnesium fluoride may be deposited by sputtering but requires a reactive atmosphere including fluorine or hydrogen fluoride.
The Rock system is simple as it has only four layers. However, since it requires a relatively high refractive index material, such as titanium dioxide, a, high sputtering rate is difficult to obtain. Typically, the deposition rate for titanium dioxide reactively sputtered from titanium is only one-quarter that of silicon dioxide reactively sputtered from silicon. For a Rock system using titanium dioxide and silicon dioxide, the deposition of titanium oxide would take about four times longer than the deposition of silicon dioxide.
The Rock system may require approximately equal thicknesses of titanium dioxide and silicon dioxide. Silicon dioxide may be sputtered four times faster than titanium dioxide. In order to operate at optimum speed, a glass coater may require four times as many sputtering cathodes for titanium dioxide as for silicon dioxide. However, the coater may not have enough chambers to accommodate all of these titanium dioxide cathodes. Thus, the deposition rate for the silicon dioxide will have to be reduced to "keep pace" with the deposition rate of the titanium dioxide. This reduces output and increases production costs.
It is widely believed that materials which can be deposited at high rates by DC reactive sputtering have relatively low refractive indices. Deposition rate comparisons may be slightly inconsistent from source to source. The type of machine and cathode used may also influence the results. The following approximate rate comparisons serve to illustrate the generalization. The refractive index values cited are the approximate values at a wavelength of about 520 nm. Titanium dioxide has a refractive index of about 2.35, and tantalum oxide (Ta.sub.2 O.sub.5) has a refractive index of about 2.25. Tantalum oxide may be deposited at about twice the rate of titanium dioxide. Zirconium oxide (ZrO.sub.2) has a refractive index of about 2.15 and may be deposited at about twice the rate of titanium dioxide. Tin oxide has a refractive index of about 2.0 and may be deposited about ten times the rate of titanium dioxide. And zinc oxide has a refractive index of about 1.90 and may be deposited about fifteen times the rate of titanium dioxide.
A layer of a material such as zinc oxide or tin oxide in an antireflection coating may be included to cause the coating to be electrically conductive. Zinc oxide may be made conductive by doping it with aluminum, and tin oxide may be made conductive by doping it with antimony. The refractive index of the doped materials remains about 2.0. Other transparent conductive materials having a refractive index of about 2.0 include Cadmium Tin Oxide (Cadmium Stannate) and Indium Tin Oxide (ITO).
A problem of using high index materials in a Rock-type antireflection coating is that such materials are relatively slow to deposit and impart a large quantity of heat to the substrate being coated. Although DC reactively sputtered materials such as titanium dioxide, niobium pentoxide, or tantalum pentoxide, or similar materials have an indices of refraction higher than 2.2, these materials impart so much heat to the substrate that only substrates having a high melting point, such as glass, are suitable. A large amount of heat is transferred to the substrate because the deposition process is slower and therefore there is more time for heat to be transferred, and because the materials are harder and may only be sputtered at higher temperatures. As a result, it is difficult to deposit antireflection coatings on temperature sensitive substrates such as plastic. A temperature sensitive substrate may be said to be a substrate which has a melting point or ignition point lower than the softening point of glass. A glass that is commonly used in anti-reflective coatings is soda lime float glass, which has a softening point of about 620 degrees centigrade.
Accordingly, an object of the present invention is to provide an antireflection coating for a temperature sensitive substrate, such as plastic.
Another object of the present invention is to provide an antireflection coating for economical, high volume production in an in-line reactive sputtering apparatus.
A further object of the present invention is to provide an antireflection coating utilizing materials which may be quickly sputtered in order to reduce the amount of heat transferred to the substrate.
Yet another object of the present invention is to provide an antireflection coating wherein at least one of the layers is tin oxide, indium oxide, zinc oxide, tin-doped indium oxide, bismuth-tin oxide, zinc-tin oxide or antimony-doped tin oxide.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly point out in the claims.